<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>DocDynamic underlineument</title>
    <style>
      @keyframes underlineAnimation {
        0% {
          background-size: 0 2px;
        }
        100% {
          background-size: 100% 2px;
        }
      }
      span {
        background: linear-gradient(to right, #ec695c, #61c454) no-repeat right bottom;
        background-position-x: left;
        animation: underlineAnimation 2s infinite;
      }
    </style>
  </head>
  <body>
    <span>CSS才是你永远也学不会的语言</span>
  </body>
</html>
